Known from German Patent DE 32 12 434 C2 is to monitor the fill level of a medium by detecting whether an electrical contact is present via the conductive medium between a sensor electrode and the wall of a conductive container or a second electrode. Since, as a function of the medium to be monitored, accretions often form on the measuring probe, a guard electrode is used. Such surrounds the sensor electrode coaxially and lies at the same electrical potential as the sensor electrode. Depending on the character of the accretion, this embodiment has the problem of suitably producing the guard signal.
In German Patent DE 10 2006 047 780 A1, a fill level measuring probe is described, which over a large measuring range is insensitive to accretion formation. In the known solution, an amplifying unit and a limiting element are provided, wherein the limiting element is arranged between the output of the amplifying unit and the guard electrode. The guard electrode is supplied with a guard signal via the amplifying unit and the limiting element, which is e.g. an ohmic resistance. The sensor electrode is supplied analogously with the operating signal. An evaluation unit monitors the fill level based on the electrical current signal tappable on the sensor electrode and the operating signal and/or the guard signal. The amplifying unit, which produces the guard signal, is limited by the limiting element. The signal limited in its amplitude is given to the sensor electrode as exciter signal. Tapped from the sensor electrode is then an electrical current signal, which in combination with the operating signal or the guard signal is taken into consideration for the purpose of monitoring the fill level.
Known from German Patent DE 10 2008 043 412 A1 is a fill level switch having a memory unit, wherein stored in the memory unit are limit values for different media located in a container. In the case of exceeding or subceeding the limit value matched to the medium, a switching signal is produced. Especially, the limit value for the measured value can be so established in reference to the medium located in the container that accretion formation does not influence reliable switching. Since accretion formation corrupts the measurement signal and, thus, falsely indicates an incorrect process variable, the limit value (which determines the switching point) is preferably so placed that it lies outside of the range for the measurement signal, which is attainable because of the accretion. The apparatus can, in such case, be embodied as a capacitive or as a conductive, fill-level measuring device. Since the apparatus can automatically adjust to alternating media (e.g. in the context of cleaning procedures such as CIP and SIP processes) in the container by ascertaining, respectively calculating, the optimal switching point from the registered properties of the medium, complex adjustment procedures, which usually are necessary in the case of an alternation of the medium, can be omitted.
Fill level detection by means of a conductive measuring method reaches its limits when the medium to be monitored has virtually no electrical conductivity (<1 μS/cm) or only a very low conductivity. A change of the conductivity of the medium relative to the conductivity of air is then too small, in order still safely to be able to be registered by the measuring electronics. Media difficult to be monitored with a conductive measuring method include e.g. distilled water, molasses and alcohols. Still problematic are media with an electrical conductivity of less than 1 μS/cm and a dielectric constant (DK) of less than 20. Falling in these regions are especially oils and fats.
Described in German Patent DE 102013102055.0, filed on Mar. 1, 2013, published on Sep. 18, 2014, is a method for monitoring a predetermined fill level of a medium in a container. The method includes the following method steps: a fill level measuring probe is operated in a conductive operating mode as the conductive fill level measuring probe and in a capacitive operating mode as a capacitive fill level measuring probe; the conductive operating mode and the capacitive operating mode are alternately operated; based on the measured values of the two operating modes, it is ascertained, whether the predetermined fill level of the medium in the container has been achieved; a report is generated, when the predetermined fill level is exceeded and/or subceeded.